The present invention pertains to optical modulators. More particularly, the present invention relates to micro electromechanical systems (MEMS)-based optical modulators that rely on optical interference as a principle of operation.
Some optical modulators are capable of varying the intensity of an optical signal. This intensity variation can be achieved using optical interference principles. Modulators relying on this operating principle typically incorporate an optical cavity that is defined by two spaced surfaces having appropriate indices of refraction. Varying the size of the gap between the two surfaces alters the reflectivity of the optical cavity.
Optical modulators that operate in this fashion have been built using MEMS technology. For example, FIGS. 1-3 depict a MEMS-based optical modulator 100 that is disclosed in U.S. Pat. No. 5,751,469.
Referring now to FIG. 1 (cross-sectional view) and FIG. 2 (plan view), modulator 100 includes a membrane 104 that is suspended above substrate 102 by support layer 106. Membrane 104 and substrate 102 are parallel to one another and separated by gap 108. In modulator 100, membrane 104 completely overlaps optical cavity 110 which is defined laterally by the perimeter of a circular opening in support layer 106 and vertically by membrane 104 on top and substrate 102 on the bottom. Membrane 104 overlaps the optical cavity in the same manner as a drum-head overlaps the body of a drum.
As a consequence of the circular shape of optical cavity 110, the unsupported portion of membrane 104 (i.e., the portion of the optical cavity) is, of course, circular. As described in U.S. Pat. No. 5,751,469, this configuration advantageously significantly reduces stress that would otherwise concentrate in the narrow membrane support arms that are typically used to support the membrane in other prior art designs (see, e.g., U.S. Pat. No. 5,500,761).
Membrane 104 advantageously has a plurality of holes 112. In the embodiment depicted in FIG. 2, holes 112 are radially arranged, although other configurations can suitably be used. Holes 112 damp membrane vibration and are also used during fabrication to deliver etchant, typically HF acid beneath membrane 104 to create optical cavity 110. Holes 112 are located in membrane 104 outside of a centrally located xe2x80x9coptical windowxe2x80x9d 114 that receives optical signal 120 from an optical waveguide, such as an optical fiber (not shown).
In operation, membrane 104 moves toward substrate 102 (see FIG. 3) under the action of an actuating force. And, as it does so, the size of gap 108 decreases, altering optical properties of optical cavity 110. In particular, the reflectivity of the device changes. For a membrane having a thickness equal to one quarter of a wavelength of the incident optical signal, as measured in the membrane (hereinafter xe2x80x9cquarter-wavexe2x80x9d layer or membrane or xe2x80x9cxcex/4xe2x80x9d), a relative maxima in reflectivity occurs when gap 108 is equal to odd integer multiples of one-quarter of the operating wavelength (xe2x80x9chigh reflectivity statexe2x80x9d). That is, relative maxima occur at:
RMax=mxcex/4xe2x80x83xe2x80x83[1]
where: xcex is the operating wavelength of the modulator; and
m equals 1, 3, 5 . . .
Similarly, relative minima in reflectivity occur when gap 108 is equal to zero or an even integer multiple of one-quarter of the operating wavelength (xe2x80x9clow reflectivity statexe2x80x9d):
RMin=mxcex/4xe2x80x83xe2x80x83[2]
where: xcex is the operating wavelength of the modulator; and
m equals 0, 2, 4 . . .
The maximum contrast (see below; contrast=RMax/RMin) is obtained when, in one state of the modulator, the size of gap 108 results in a reflectivity maxima and, in the other state, the size of gap 108 results in a reflectivity minima. Consequently, in a xe2x80x9cquiescentxe2x80x9d or xe2x80x9cnon-actuatedxe2x80x9d state, as those terms are used herein, membrane 104 has a first position wherein the size of the gap is such that either an RMax or RMin condition is met. In an xe2x80x9cactuated state,xe2x80x9d as that term is used herein, membrane 104 moves to a second position nearer substrate 102. Again, for maximum contrast, membrane 104 moves through a distance xcex/4 when actuated.
In practice, the difference in size of gap 104 in the non-actuated and actuated states is often less than xcex/4 since the membrane xe2x80x9csnaps downxe2x80x9d to the substrate if membrane deflections greater than about thirty to thirty-five percent (relative to the size of the gap in the unbiased state) occur. Consequently, rather than specifying, for example, that the membrane moves between a non-actuated position of 3xcex/4 to an actuated position of 2xcex/4, a more conservative design will specify that the membrane moves between a non-actuated position of about 0.7xcex, to an actuated position of 2xcex/4. In the former design, snap down will probably occur since the membrane deflects an amount equal to: (3xcex/4xe2x88x922xcex/4)/(3xcex/4) or 33 percent. In the conservative design, snap down is avoided since the membrane deflects less than about thirty percent: (0.7xcexxe2x88x922xcex/4)/(0.7xcex)=28.6 percent.
In some embodiments, the actuating force for moving the membrane is an electrostatic force that is generated by creating a potential difference across substrate 102 and membrane 104. To that end, membrane 104 and substrate 102 are suitably conductive, or otherwise include a region of metallization or doping to provide such conductivity. In modulator 100 depicted in FIGS. 1-3, the electrostatic actuating system includes contact 116, which is in electrical contact with membrane 104 and controlled voltage source 222, and contact 118, which is in electrical contact with substrate 102 and controlled voltage source 222.
The performance of modulator 100 can be gauged using several parameters. Once such parameter is xe2x80x9ccontrast,xe2x80x9d which, as that term is used herein, is the ratio of maximum reflectance to minimum reflectance for the modulator. Another important performance parameter is the theoretical xe2x80x9cinsertion loss,xe2x80x9d which, as used herein, is one hundred minus the maximum reflectance of the modulator. A third performance parameter is xe2x80x9cbandwidth,xe2x80x9d which for the purposes of the present Specification, means the range of wavelengths over which an acceptable amount of contrast is obtained. These performance parameters of optical modulator 100 are dependent upon certain physical characteristics of the modulator such as the refractive indices of membrane 104 and substrate 102, the thickness of the membrane 104 and the size of gap 108.
A modulator possessing high contrast, low insertion loss and a wide bandwidth is desirable. But neither modulator 100, nor other prior art MEMS-based optical modulators, possess the full measure of all of these characteristics. It is known, however, that these characteristics can be traded-off, as desired. The trade-off among performance parameters is accomplished by manipulating the aforedescribed physical characteristics. TABLE I provides a summary of the manner in which modulator physical characteristics have been manipulated in the prior art to achieve a desired modulator performance. Abbreviations used in the table include: nm for the refractive index of the membrane, n, for the refractive index of the substrate, L1, L2, etc., indicates a first layer of the membrane, second layer of the membrane, etc.
Consider, for example, a traditional Fabry-Perot cavity modulator having a membrane and substrate implemented as multi-layer dielectric mirrors having high and equal reflectivity. Such a modulator can achieve a very high reflectivity (i.e., low insertion loss), with theoretical device reflectivity exceeding 90 percent. But, the traditional Fabry-Perot cavity modulator typically exhibits a rather narrow operating bandwidth of about 5 nanometers around a center operating wavelength.
U.S. Pat. No. 5,500,761 addresses the narrow bandwidth drawback of the traditional Fabry-Perot cavity by specifying a quarter wave membrane having a refractive index that is approximately equal to the square root of the refractive index of the substrate. Opposite phase mirrors (i.e., 180 degree phase deviation) are used to obtain maximal contrast (i.e., zero reflectivity in the minimum reflectivity state).
But the maximum reflectivity of the modulator disclosed in U.S. Pat. No. 5,500,761 is less than that of a traditional Fabry-Perot. So, this modulator provides wide bandwidth (and high contrast) at the expense of increased insertion loss.
The proviso in U.S. Pat. No. 5,500,761 concerning the refractive index of the membrane is somewhat problematic. In particular, the logical materials choice for the membrane-silicon nitride-exhibits undesirable mechanical properties (i.e., the intrinsic stress is too high) at the desired refractive index (e.g., 1.87 if silicon is used as the substrate).
U.S. Pat. No. 5,825,528 addresses the refractive index limitation of U.S. Pat. No. 5,500,761 with a phase-mismatched modulator. In the phase-mismatched modulator, the phase of the reflectivity of the membrane and the phase of the reflectivity of the substrate deviates by more than 180 degrees. Relaxing the phase requirement advantageously relaxes the requirement that the membrane must have a refractive index that is equal to the square root of the refractive index of the substrate.
U.S. Pat. No. 5,870,221 discloses that contrast ratio can be traded off to obtain a modulator having both low insertion loss and relatively broad operating bandwidth. According to the ""221 patent, in a modulator having unequal reflectivity mirrors, the operating bandwidth is determined by the lower reflectivity mirror. Consequently, a modulator characterized by the broad operating bandwidth of the ""761 patent, but the low insertion loss of traditional high finesse Fabry-Perot cavity, is obtained by using a low finesse mirror as the substrate and a higher finesse mirror as the membrane. According to U.S. Pat. No. 5,870,221, the membrane has at least one layer that has a refractive index that is about equal to the refractive index of the substrate. Furthermore, unlike the ""761 patent, the membrane is not a quarter-wave layer (i.e., has a thickness other than a quarter of a wavelength of the incident optical signal).
The response (i.e., modulator reflectivity as a function of applied voltage) of the aforementioned MEMS-based optical modulators is nonlinear when considered over their full operating range. While such non-linearity is typically not objectionable for digital applications, a linear operating characteristic is often strongly preferred for analog applications (e.g., cable television, standard telephony and radio applications). U.S. Pat. No. 5,838,484, manipulates physical characteristics to provide a modulator with a linear operating characteristic.
While the prior art discloses some ways to manipulate modulator physical characteristics to achieve specific performance goals, further teachings along these lines will be of benefit to the art.
In accordance with the present teachings, the physical characteristics of MEMS-based optical modulators are manipulated in new ways to achieve desired performance goals. For example, in one embodiment, the present optical modulators trade contrast ratio (i.e., accept a relatively low contrast ratio) for low insertion loss and wide bandwidth.
The present modulators advantageously comprise a single layer, quarter wave membrane that is suspended over a substrate. In some embodiments, the refractive index, nm, of the membrane is in a range 1.1ns0.5xe2x89xa6nmxe2x89xa61.4ns0.5. The membrane is movable toward the substrate when actuated. And as the membrane moves, the reflectivity of the modulator changes.
In comparison with some prior MEMS-based optical modulators, such as those that restrict the refractive index, nm, by the proviso that nm=ns0.5, the present optical modulators advantageously exhibit lower insertion loss. But in comparison to some other MEMS-based modulators, the present modulators simply provide equivalent performance. To the extent that the present modulators are no better, in terms of performance, than some earlier designs, the inventive physical configuration is simpler to fabricate. But in all cases, the present modulators achieve low insertion loss and wide bandwidth via a different physical configuration (e.g., membrane characteristics, etc.) than prior art MEMS-based optical modulators.
In further embodiments, the present modulators use a germanium substrate. Modulators having germanium substrates can achieve higher reflectivity than modulators having silicon substrates. In accordance with the present teachings, a protective layer is disposed on the germanium substrate. The protective layer protects the germanium layer from HF etchant that is typically used during the MEMS fabrication procedures.
These and other features of the invention will become more apparent from the following Detailed Description when read in conjunction with the accompanying Drawings, in which like elements have like reference numerals.